Keep your Community Folder with your addons, copy and paste it somewhere.
Any Markeplace items you may have purchased will be able to be reinstalled.
Steam files are located in these locations, make sure you have deleted any remaining after the uninstall:
C:\Program Files (x86)\Steam\steamapps\common\MicrosoftFlightSimulator
C:\Users\YourUsername\AppData\Roaming\Microsoft Flight Simulator\Packages